* Difficulty Level :Medium
* Last Updated : 11 May, Open Shortest Path First is a link-state protocol that is used to find the best path between the source and the destination router using its own shortest path first algorithm. It comes under Link State Routing Protocol. It automatically decides the path or table and is used to send router packets with auto path decision and send hello packets every 10 seconds.
It is one of the Interior gateway protocols the protocol that aims at moving the packet within a large autonomous system. works on protocol number 89 and uses AD value 110.
Criteria to form neighbourship in OSPF:
* Should be present in the same area.
* The subnet mask should be the same.
* Authentication must match.
Area in OSPF:
An area is a logical and structured way of grouping contiguous routers. But the condition is all the routers must be in the same area and have the same topology path. OSPF areas are used to optimize the path and the routing tables on the routers are reduced. So very less time is required to run the algorithm.
OSPF Area Rules:
* All the routers maintain the same database.
* Any change impacts all the routers.
* At least one area Border router
* The area is a logical grouping Router.
Issues with Large OSPF Network:
* The link-state database gets larger.
* The routing table gets larger.
* Increased memory and CPU load on routers.
* The number of LSAs flowing on the network will increase
OSPF Hierarchical Routing:
* Minimizes the size of the database.
* Restrict any changes within that area.
* Routers in the same area participate in the Algorithm.
There are five types of OSPF areas, which are as follows:
The backbone area is area 0. It plays the role of the central node in the OSPF network and the link information of other areas is transmitted through area 0. so generally OSPF area consists of all networks in area ID 0.0.0.0 which are attached to the routing devices.
In this type of area, OSPF packets can be normally transmitted and they can support up to 1,2,3,4, and 5 LSAs(Link State Advertisements). Ensure optimal routing since all routers know about all routes
As the name defines this type of area does not accept external routes and does not accept the link information of the other routers outside the area. these areas do not receive type 4 or 5 LSAs from their ABRs(Area Border Router).
Totally stubby area:
These are completely stub areas same as sub-area this type of area does not accept external routes and does not accept the link information of the other routers outside the area. It should be noted here that since the default route is sent using Type 3 LSAs, the complete stub area does not allow ordinary Type 3 LSA packets, but it supports this type of LSA with default routes. This area supports Type 1, Type 2 LSAs, and Type 3 LSAs with default routes.
Not so Stubby area:
It also has to send external routes to other areas. This area supports 1, 2, 3, and 7 LSAs. An NSSA makes use of type 7 LSAs.The functionality of NSSA is to designate a normal (stub) NSSA, all routers in the area must be so configured: This allows an ASBR to advertise external links to an ABR, which converts the type 7 LSAs into type 5 before flooding them to the rest of the OSPF domain.